An organic coating film is coated on the inner surface of a metal can for the purpose of preventing corrosion of the metal material of the can and of not deteriorating the taste and flavor of the content in the can, and it is needed to be non-toxic, to be durable to heat sterilization, to involve dissolution of only a small mass of constitutive components and to have good adhesion and workability.
Beverage cans and food cans are grouped into so-called three-piece cans and two-piece cans from the methods of manufacturing them. Recently, two-piece cans are increasing due to increase of the demand for beer, juice, soda drinks, etc.
A two-piece can is called a side-seamless can, which is formed by drawing or ironing a metal material such as an aluminium sheet, tinplate or tin-free steel sheet to give a cup composed of a seamless body and a bottom as seamlessly integrated with the body. It is grouped into a drawn and ironed can (DI can), a drawn can (DR can) and a drawn and re-drawn can (DRD can) from the forming methods.
For coating a two-piece can with an organic coating film, in general, a method of spray coating a shaped can with a coating agent is much used. Recently, in addition, there are also known a method of previously laminating a synthetic resin film on a metal material and a method of using a metal sheet as precoated with a coating agent such as a vinyl organosol, epoxy-phenolic or acrylic material, before shaping it into a can.
However, since the former method needs a step of laminating a film such as polyethylene terephthalate on the metal base with an adhesive, the manufacture cost thereof is high. Therefore, the latter method is desired. However, since the vinyl organosol coating agent consists essentially of a vinyl chloride polymer, it involves problems of insanitation, necessity of recycling of used cans and environmental pollution. The other epoxy-phenolic coating agent and acrylic coating agent could not follow the elongation or deformation of the metal base in the shaping step of manufacturing cans so that the coated film would be cracked or peeled off from the base and, as a result, the corrosion resistance of the film as previously coated on the metal sheet base is noticeably decreased by the process of shaping it into a can.
In the process of manufacturing DI cans and DRD cans, the previously coated film is often scratched mechanically to cause peeling of the coated film and decreasing of the adhesion of the same to the metal base. The defects become noticeable in the heat sterilization step to be effected after charging of a product into the shaped can. As a result, the can could not be used as a food can or beverage.
A polyester resin has high adhesion to metal and has few problems of insanitation, and use of it as a coating agent to be applied to the inner surface of a metal can has heretofore been investigated. However, the current situation is such that a polyester resin coating agent which is durable to the above-mentioned severe shaping step and which has high resistance to blistering and high resistance to whitening in heat sterilization is unknown up to the present.